This invention relates to an amplifier circuit, and particularly to an amplifier circuit suitable for use as an active filter circuit, which can be easily formed into an integrated circuit (IC).
When an attempt is made to allow an IC (integrated circuit) to comprise such circuits including filters as integrator and phase shift circuits, it is important how these filters should be formed in the IC so as to decrease the number of components to be externally connected. As filters for use in an integrated circuit, active filters are usually used. However, disadvantages of a conventional active filter are:
(1) it is difficult to form its resistors and capacitors with accurate values, causing errors in its cut-off frequency which is determined by the product of these values; and PA0 (2) it is difficult to form a circuit with a lower cut-off frequency because resistors and capacitors are limited to have relatively low values.
Conventional active filters are disclosed in JP-A-55-45224 and JP-A-55-45266.